This invention relates to biaxially textured buffer layers on metal substrates. More specifically, the invention relates to a process for depositing a protective layer over a biaxially textured substrate and the resulting composition.
Biaxially textured buffer layers on metal substrates are potentially useful in electronic devices where a biaxially textured electronically active layer is desired. The electronically active layer may be a superconductor, a semiconductor, or a ferro-electric material.
For example, the next generation of superconducting wire to be used for power transmission lines is likely to have a multi-layer structure. Such deposited conductor systems consist of a metal substrate, buffer layer, and a superconducting layer. The metal substrate, such as Ni and its Ni alloys, provides flexibility and support for the wire. Buffer layers, such as metal oxide buffer layers including cerium oxide (CeO2) and yttria-stabilized zirconia (YSZ), comprise the next layer and serve as chemical barriers between the metal substrate and the top layer, the high-temperature superconductor.
Many device applications require a good control of the grain boundary character for the materials that comprise the device. For example, grain boundary character is very important in high temperature superconductors. The effects of grain boundary characteristics on current transmission across the boundary have been very clearly demonstrated for YBa2Cu3Ox (Y123). For clean, stoichiometric boundaries, Jc(gb), the grain boundary critical current, appears to be determined primarily by the grain boundary misorientation.
The dependence of Jc(gb) on misorientation angle has been determined (Dimos et al., Phys. Rev. Lett. 61, 219, 1988; Dimos et al., Phys. Rev. B 41, 4038, 1990) in Y123 for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case, high angle boundaries were found to be weak-linked. The low Jc observed in randomly oriented polycrystalline Y123 can be understood on the basis that the population of low angle boundaries is small and that frequent high angle boundaries impede long-range current flow.
Recently, the Dimos experiment has been extended to artificially fabricated [001] tilt bicrystals in Tl2Ba2CaCu2O8 (Cardona et al., Phys. Lett., 62 (4), 411, 1993), Tl2Ba2Ca2Cu3Ox (Kawasaki et al., Appl. Phys. Lett., 62 (4), 417 1993), TlBa2Ca2Cu2Ox (Nabatame et al., Appl. Phys. Lett. 65 (6), 776 1994) and Nd1.85Ce0.15CuO4 (Kawasaki et al.). In each case it was found that, as in Y123, Jc depends strongly on grain boundary misorientation angle. Although no measurements have been made on Bi2Sr2Ca2Cu3Ox (Bi-2223), data on current transmission across artificially fabricated grain boundaries in Bi-2212 indicate that most large angle [001] tilt and twist boundaries are weak links, with the exception of some coincident site lattice (CSL) related boundaries (Tomita et al., Jpn. J. Appl. Phys., 29, L30, 1990; Tomita et al., Jpn. J. Appl. Phys., 31, L942, 1992; Wang et al., Physica C, 230, 189, 1994). It is likely that the variation in Jc with grain boundary misorientation in Bi-2212 and Bi-2223 is similar to that observed in the well characterized cases of Y123 and Tl-based superconductors.
To fabricate high temperature superconductors with very high critical current densities, the grains are therefore preferably aligned. This has been shown to result in significant improvement in the superconducting properties of YBCO films (Iijima et al., J. of Appl. Phys., 74, 1905, 1993; Reade et al., Appl. Phys. Lett., 61, 2231, 1992; Wu et al., Appl. Phys. Lett., 65, 1961, 1994). Most preferably, the grains should be aligned both out-of-plane with respect to the substrate (c-axis oriented) and in-plane with respect to the substrate (a-b alignment). To achieve this alignment, high Tc superconductors have generally been deposited on (100) oriented single-crystal oxide substrates. However, single-crystal substrates are generally too expensive and have poor mechanical properties. As such, single-crystal substrates are presently unsuitable as practical conductors.
A method to develop practical coated conductors is disclosed in U.S. Pat. No. 5,741,377 (""377) by Goyal et al. This method called RABiTs, an abbreviation for xe2x80x9crolling assisted biaxially textured substrates,xe2x80x9d uses roll-texturing of metal to form a metallic tape with a {100} less than 001 greater than  cubic structure. However, if the metal is nickel or a nickel alloy, a buffer layer between the metal substrate and the ceramic superconductor is necessary to prevent interdiffusion of the ceramic superconductor and the metal substrate and also to prevent the oxidation of nickel substrate during the deposition of the superconducting layer. Useful buffer layers include cerium oxide, yttrium stabilized zirconia (YSZ), strontium titanium oxide, rare-earth aluminates, other perovskites, various rare-earth oxides, and nitrides.
To achieve high critical current densities, it is important that the biaxial orientation be transferred from the substrate to the superconducting material. As stated, a biaxially textured metal substrate can be provided by the method disclosed in the ""377 patent. The processes that are currently used to grow buffer layers on metal substrates and achieve this transfer of texture include pulsed laser deposition, sputtering, electron beam evaporation and sol-gel techniques. Researchers have recently used such techniques to grow biaxially textured YBa2Cu3Ox (YBCO) films on metal substrate/buffer layer samples that have yielded critical current densities (Jc) approaching 3xc3x97106 A/cm2 at 77xc2x0 K (A. Goyal, et al., J. of Superconductivity, vol. 11, No. 5, 1998).
A further consideration during the fabrication process is the undesirable oxidation of the metal substrate, for example when using Ni. If the Ni begins to oxidize, the resulting NiO is likely to grow in the (111) orientation regardless of the orientation of the Ni (J. V. Cathcart, et al., J. Electrochem. Soc. 116:664, 1969). This (111) NiO orientation adversely affects the growth of biaxially textured layers and will be transferred, despite the substrate""s original orientation, to the following layers.
One area of difficulty in using Ni as the substrate is due the ferromagnetic properties of Ni. This causes significant problems in practical applications involving superconductors, particularly in terms of AC losses. Biaxially textured Ni alloy substrates having reduced magnetism have been suggested as suitable substrates for the fabrications of HTS conductors (A. Goyal et al., U.S. Pat. No. 5,964,966; A. Goyal et al., U.S. Pat. Nos. 5,958,599 and 5,898,020). However, these alloy substrates have elements that form native oxide layers with undesirable crystallographic orientations. Whereas the formation of NiO can easily be prevented in pure Ni substrates, the formation of oxides of alloying elements, such as Cr, is very difficult to prevent. Typically, with Ni substrates, the deposition of oxide buffer layers is done under conditions where the formation of NiO is thermodynamically unstable and the formation of the desired oxide buffer layers is thermodynamically favorable. This is typically accomplished by performing the deposition under reducing conditions using a forming gas with oxygen provided by water vapor. It is difficult, however, to find such conditions for preventing oxidation of alloying elements of Ni used to form the non-magnetic or reduced magnetism substrates.
It is an object of this invention to provide a new and improved method for fabricating alloy and laminated structures having excellent transference of epitaxial texture through a multilayer structure.
It is another object of the invention to provide a method to produce epitaxial superconductors on a non-magnetic or reduced magnetic substrate and laminated structures having epitaxial texture.
It is yet another object of the invention to provide a method and a product of that method to prevent or significantly reduce the surface oxidation of the non-magnetic or reduced magnetic substrates, on which epitaxial buffer layers are deposited.
It is a further object of the invention to provide an epitaxial textured superconducting structure having a Jc of greater than 100,000 A/cm2 at 77xc2x0 K and self-field.
These and other objects of the invention are achieved by a laminate article comprising a substrate and a biaxially textured protective layer over the substrate. The substrate can be biaxially textured and also have reduced magnetism relative to Ni. The substrate can be selected from the group consisting of nickel, copper, iron, aluminum and alloys containing any of the foregoing. Layers of superconducting materials such as, but not limited to YBCO, and buffer layers such, as but not limited to, CeO2, YSZ, LaAlO3, SrTiO3, Y2O3, RE2O3, SrRuO3, LaNiO3, La2ZrO3 and TiN and combinations of the foregoing can be deposited over the protective layer.
The protective layer can be selected from the group consisting of nickel, silver, gold, platinum, and palladium and alloys containing any of the foregoing. The protective layer is also non-oxidizable under conditions used to deposit the buffer layers. A method of forming the laminate article is also disclosed.